Given specific rewards, how can I calculate the returns for each time step?
Perfect. 

To back up your intuition about there not being a $G_5$, refer to the definition of discounted return in the periodic case (3.11). 
$$G_t \doteq \sum_{k=t+1}^T \gamma^{k-t-1} R_k$$

You'll see that $G_5$ would be written as a sum with no terms in it, since $T=5$.